Basic questions

Answered Question
Mar 10th, 2008

Hi friends,


Just two basic questions:


1. When i say


router ospf 100

passive-interface default

netw x.x.x.x y.y.y.y area z


What exactly is the purspose of this network statement. Does this statement attempt to establish adjacency with other devices in the same network or does it advertise this network to its adjacent neighbours? Also does this statement override the passive-interface default command given earlier?


Regarding the second question, if there is a L3 switch with interface IP 10.1.16.1/30 (vlan 16) and there are switchports configured for vlan16, no vlan (vlan 1), trunk ports, vlan 15.


Now, when this switch sends traffic to 10.1.16.2, to which switchport does the switch sends its traffic through?

I belive that it will send traffic through vlan16 port, trunk port and vlan 1 port. The vlan15 port will not be used to send this traffic? Hope i am right


Thanks a lot

Gautam



Correct Answer by Jon Marshall about 8 years 11 months ago

Gautam


If a packet is destined for vlan 16 then it could be sent to a port allocated to vlan 16 or a trunk port. It will not be sent to a port in vlan 15 or a port in vlan 1. Ports in vlan 1 are still in a vlan, just the default vlan.


Does this answer your question ?


Jon

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Jon Marshall Tue, 03/11/2008 - 00:36

Hi Guatam


The network statement under OSPF/EIGRP/RIP tells the router which interfaces to run the routing protocol on. So if you had


router ospf 100

network 192.168.10.0 0.0.0.255 area 0


and an interface

int fa0/0

ip address 192.168.10.1 255.255.255.0


then the router will run OPSF on this interface. The network statement does not tell the router which networks to advertise.


For completeness with BGP a network statement does tell the router which networks to advertise.


Not sure about 2nd question. Depends on where the source machine is ie. if the source is in vlan 15 then the vlan 15 interface would be used.


When you talk about interface IP of 10.1.16.1 are you referring to an SVI or a routed port ?


Jon

Correct Answer
Jon Marshall Tue, 03/11/2008 - 08:28

Gautam


If a packet is destined for vlan 16 then it could be sent to a port allocated to vlan 16 or a trunk port. It will not be sent to a port in vlan 15 or a port in vlan 1. Ports in vlan 1 are still in a vlan, just the default vlan.


Does this answer your question ?


Jon

mohammedmahmoud Tue, 03/11/2008 - 01:47

Hi Gautam,


Let me elaborate on the passive-interface part, passive-interfaces will not send Hellos and thus no adjacencies can be formed on these interfaces, and thus if you use "passive-interface default" this router will never have adjacencies (since this will make all the interfaces passive), and you need to disable passive-interface on the interface(s) that you need to have adjacency on for your OSPF network to operate.


Another issue, is that when configuring an interface as passive with OSPF, the LSA of this interface will be sent over adjacencies on other OSPF enabled interface(s) on the router, but the interface itself won't form adjacency, the confusing part here, is that you might see this as if the network command told the router which network to advertise, but as illustrated, no it didn't, it told the router to operate OSPF on this interface but not to form adjacency, just send its LSA.


I hope that i've been informative.


BR,

Mohammed Mahmoud.

Richard Burts Tue, 03/11/2008 - 05:27

Gautam


Jon and Mohammed have given good answers to parts of your question. Let me address another part of your question. You ask which port will the switch use when it sends traffic to 10.1.16.2? The answer is that the switch will ARP for 10.1.16.2 to learn the MAC associated with the IP address and then will send out the port on which this MAC is learned.


HTH


Rick

gautamzone Tue, 03/11/2008 - 08:11

Thanks Rick. But will it send out the port that is configured with vlan 15? I think that only trunk ports, ports configured with no vlan (vlan 1) and vlan 16 ports will only be receiving the traffic from 10.1.16.1 (SVI for Vlan 16).



gautamzone Tue, 03/11/2008 - 08:17

Thanks Mohammed. So you are saying that if two L3 devices are adjacent OSPF neighbours, then if i create a SVI on one router and say passive-interface on the router, this new SVI's network will still be advertised to the adjacent router(even if i dont mention the network command)?


Another question that has sprung from my mind is :


What is the difference between giving "no passive-interface" command and issuing the network command for the particular interface?



Richard Burts Tue, 03/11/2008 - 08:30

Gautam


As I explained, the switch will send the traffic through the port on which it has learned the MAC address associated with the IP address. That is the fundamental and correct answer. Since the address is in the subnet associated with VLAN 16 I would expect that the port would be a port associated with VLAN 16 (or perhaps a trunk port is the destination port in VLAN 16 were on another switch). If the traffic goes to a port in VLAN 15 or in VLAN 1 then something is wrong in the way that the network is set up.


As far as the network statement in OSPF and passive interface is concerned, it is important to understand that there is no relationship between active or passive interface and the network statement. As was explained in a previous post the function of the network statement is to tell the routing protocol which interface to include in the routing protocol. If you do not put a network statement there will be no interface included in the routing protocol (and its subnet will not be advertised unless you do redistribute connected).


Once you have used a network statement to include an interface in OSPF then there is the question of whether the interface will be active or passive. If the interface is active then it will attempt to find neighbors and to build a neighbor relationship. If the interface is passive then it will not build any neighbor relationship.


The normal default for OSPF interfaces is to be active. Your question included the configuration command passive-interface default. This changes the default behavior and will have interfaces be passive unless you specifically configure that interface to be active.


HTH


Rick

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